Doublet pulse coherent laser radar for tracking of resident space objects

Narasimha S. Prasad*1

, Van Rudd2, Scott Shald

2,

Stephan Sandford1 and Albert DiMarcantonio

2

1NASA Langley Research Center, 5 N. Dryden St., MS 468, Hampton VA, 23681

2135 S. Taylor Ave., Lockheed Martin Coherent Technologies, Inc., Louisville, CO 80027

ABSTRACT

In this paper, the development of a long range ladar system known as ExoSPEAR at NASA Langley Research

Center for tracking rapidly moving resident space objects is discussed. Based on 100 W, nanosecond class, near-IR

laser, this ladar system with coherent detection technique is currently being investigated for short dwell time

measurements of resident space objects (RSOs) in LEO and beyond for space surveillance applications. This unique

ladar architecture is configured using a continuously agile doublet-pulse waveform scheme coupled to a closed-loop

tracking and control loop approach to simultaneously achieve mm class range precision and mm/s velocity precision

and hence obtain unprecedented track accuracies. Salient features of the design architecture followed by

performance modeling and engagement simulations illustrating the dependence of range and velocity precision in

LEO orbits on ladar parameters are presented. Estimated limits on detectable optical cross sections of RSOs in LEO

orbits are discussed.

1. INTRODUCTION

The threat of orbital debris not more than five years ago might be characterized as ‘critically remote’. Today,

given the increasing numbers of space-faring nations, orbital systems and unwanted conjunctions, it would be

characterized as ‘critically imminent’ [1, 2]. The assessment of possible conjunctions with other resident space

objects (RSOs) is critical to protection of commercial, civil and Department of Defense (DoD) space assets. This is

entirely dependent on the orbit determination (OD) accuracy and the ability to predict drag and solar radiation

pressure effects on the RSOs accurately. While precise ranging to an RSO using radar or singlet pulse based laser

systems can be limited by the effects of tumbling, extremely accurate Doppler measurement is possible using a

doublet coherent laser tracking system. Addition of such tracking to the OD processing can significantly improve the

accuracy of these orbits for possible conjunctors, allowing more accurate event forecasting.

Existing technologies used to identify and track RSOs primarily include X-Band and Ka-Band radars [3], and

passive or optical telescopes [4]. These systems are limited in their abilities to simultaneously and accurately range,

track and characterize RSOs. Typical conjunction predictions are based on statistical models and mathematical

analysis, and can only estimate the probability of a collision between orbiting objects. Advanced sensors with

improved track accuracies would help improve conjunction predictions analyses.

NASA LaRC is advancing a novel long range ladar technology known as ‘ExoSPEAR’ for space surveillance

applications. ExoSPEAR is a technically innovative and operationally unique ground-based LADAR system for

aerospace observation and measurement. The ExoSPEAR ladar system architecture was developed by Lockheed

Martin Coherent Technologies (LMCT) (previously known as Coherent Technologies, Inc., CTI) under funding

from AFRL, Kirtland AFB, Albuquerque, NM for long range tracking of fast moving objects under the program

known as Range Acquisition and Tracking Laser-Radar (RATLR) (Contract # FA9451-07-C-0220). NASA

acquired this prototype system under an interagency agreement and has now re-purposed it for space exploration and

science applications. The ExoSPEAR ladar technology will enable precision measurements to accurately search,

detect, identify, classify, characterize, target, localize, and track specific resident space objects (RSO) to facilitate

removal or evasion operations.

ExoSPEAR is specifically designed to provide very high precision, short-dwell-time measurements of RSOs in

LEO and beyond. Currently, LaRC is expanding its utility and scope for space situational awareness, astrophysics

and atmospheric sensing applications from its initial objectives. Its technical capabilities include rapid RSO track

acquisition, micro-motion or vibrometry and imagery. This system had been ground tested at White Sands Missile

* narasimha.s.prasad@nasa.gov; Phone 757-864-9403; Fax 757-864-8828.

Range using static targets, spinning cones returns for testing speckle decorrelation, and retro returns suitable for

satellite tracking. With precision ranging and tracking and hence precision OD, this evolving technology enables

reducing conjunction prediction regions for ranges LEO and beyond. The proposed technology confidently

challenges traditional systems for space debris detection and monitoring, space object identification, space

situational awareness, and sub-millimeter range tracking of RSOs for solar physics, general relativity, precision

navigation.

Exo-SPEAR ladar is based on an innovative, first-in-class, patented doublet-pulse technology for very high

precision, rapid acquisition, and day-night space observation. Doublet pulse based Coherent Detection

architecture provides highly sensitive and precise tracking measurements (US Patent # 5,815,250). The

predicted performance of the baseline ladar architecture includes mm-class range resolution, mm/s class velocity

resolution and microrad angular resolution with an estimated error-covariance of ~ 1m x 5m and maximum iso-

planatic patch. The current system employs unique innovative technologies that can easily lend themselves to

orbital debris detection, characterization, and tracking. This system can provide rapid RSO track acquisition, RSO

micro-motion or vibrometry, 3D-imagery, and precision, navigation, and timing (PNT). Plans are underway to

demonstrate tracking less than 10 m2 optical cross-section (OCS) targets in LEO. With energy scaled version of

this ExoSPEAR technology combined with improved receiver electronics and large diameter telescopes, tracking 10

cm2 cross section targets in LEO as well as tracking near Earth objects (NEOs) such as meteoroids, and asteroids

may well be possible.

The basic concepts of doublet pulse based coherent ladar scheme and its advantages over direct detection

scheme along with velocity and acceleration precision estimates using typical operational parameters are described

in the reference [5]. Coherent Singlet-Pulse ladar has measurement limitations. In the case of short pulse operation,

reasonable good range resolution can be obtained but will provide poor velocity resolution and hence cannot provide

precision tracking. However, range resolution reduces for long pulse operation but velocity resolution increases.

With a single transform-limited pulse one must trade-off between range and velocity resolution. To overcome the

single-pulse limit, large time-bandwidth pulses have to be utilized. With a doublet-pulse waveform range and

velocity resolution is obtained simultaneously. Figure 1 illustrates simultaneous measurement of range and velocity

by a doublet pulse waveform based ladar.

Short PulseGood Range,

Poor Velocity

Long PulsePoor Range,

Good Velocity

Doublet PulseGood Range,

Good Velocity

Figure 1. Left: Good range resolution with single short pulses. Middle: Good velocity resolution with longer

pulsewidth operation. Right: Doublet pulse operation provides good range and velocities.

The simultaneous measurement of range and velocity using short pulsewidth doublet pulse coherent ladar technique

offers a means for precision tracking. The technique offers best of both worlds; precise range measurements from

narrow pulses, precise velocity from Doppler shift over long dwell time. The pulse separation is adjusted in real-

time to provide best possible measurement. The pulse separation cannot be too small due to poorer velocity

precision as well as too large to avoid speckle decorrelation.

Precision requirements drive system requirements. With pulsewidths of 10 ns or less, range resolution < 1 m or

one part in 106 can be achieved. For velocity measurements of < 1 cm/sec (need to hit the Cramer-Rao Lower

Bound), one part in 105 precision, doublet-pulse spacings has to be >1 msec. The agile doublet-pulse waveform

forming setup in ExoSPEAR meets all of these performance requirements. Accordingly, the overall performance

goal of the current ExoSPEAR system is to achieve high-accuracy tracking of long-range, fast-moving objects in

LEO. In this paper, the performance modeling and engagement simulation results of the ExoSPEAR system are

discussed.

2. EXOSPEAR SYSTEM ARCHITECTURE

Theory and experimentation techniques of coherent and other ladar techniques are discussed in references [6-8].

Figure 2 shows the schematic of the ExoSPEAR ladar architecture based on doublet pulse coherent ladar technique

to achieve enhanced velocity and range resolution. It comprises of two diode pumped single longitudinal mode laser

systems known as miniature Slave Oscillators (MiSO) known as MiSO1 and MiSO2 configured in master oscillator

power amplifier (MOPA) architecture. Table 1 shows specifications of each laser transmitter. In each case,

commercially available seed lasers are used to configure an arrangement known as stable master oscillator local

oscillator (SMOLO). The output is fiber coupled to a MiSO consisting of slab amplifier layout to achieve up to 60

mJ/pulse. The output from these two laser systems is combined using a Pockels cell arrangement in combination

with two Free Scanning Mirrors (FSMs) to achieve adaptive pulse separation. Monitor setup consisting of acousto-

optic modulator and receiver components are used to observe laser beam characteristics for proper alignments. The

transceiver optics arrangement with lag mirror and Transmit/Receive (T/R) switch will simultaneously transmit as

well as receive the return beam. The return laser beam, collected by a telescope, will be coherently detected using a

single pixel detector after mixing it with the local oscillator. The corresponding electronic signal is directed for

further processing.

Control SystemDrive Electronics

Doublet Pulse Control

Signal ProcessingTrack Algorithms

Data Archival

Master Oscillator

Tunable Local

Oscillator

Offset Lock Electronics

High Power Lasers

Monitor Receiver

Fiber Coupler

Fiber Amplifier

MiniaturizedSlave Oscillator 2

(MISO2)

MiniaturizedSlave Oscillator 1

(MISO1)

Return Detector

Fiber Coupler

Pockel sCell

SetupFSM2

FSM1

Lag Mirror

T/R Switch

Transceiver Optics

RF Source

Return Signal

AOM

Beam Combiner

SMOLO

Monitor Setup

Figure 2: The ExoSPEAR ladar System architecture.

Table 1: Laser transmitter specifications

Parameters Specifications

Wavelength 1.064 μm

PRF 800 Hz, (two Lasers each running at 400 Hz)

Transceiver output power > 50 W

Pulse energy > 60 mJ/pulse, 2 pulses/waveform

Pulse width < 11 ns

Pulse spacing utilized 20 ns to 1 ms

Beam quality 1.2 times Diff. Limited; PITB > 0.5

Beam Divergence (full angle) < 0.3 mrad

Spectral Transform Limited Pulse <10%

Centroid Beam Pointing Stability <1/10th Diffraction Limited

Ladar system efficiency Laser #1: 3% and Laser #2: 6%

The two electronics racks consists of Data Acquisition Computer with software for streaming data and basic

processing, Internal Alignment (IA) computer with software routine for aligning laser beams, and PXI computer

with software for controlling laser operation. A closed-loop tracking-and-control loop is utilized to resolve

ambiguity and optimize track performance. Figure 3 shows the current ExoSPEAR prototype system operational at

NASA LaRC. The entire fits into a conex container of 20 ft. x 8 ft. in length. The graphical user interface

illustrating various components of track information is shown in Figure 4.

Figure 3. Left Picture: The ExoSPEAR laser transmitters, MiSO1 and MiSO2. Middle Picture: Pockels Cell

arrangement for beam combining and adaptive doublet formation. Right Picture: The electronics rack consisting of

drive electronics and signal processing computers.

Figure 4. The coherent lidar system user interface illustrating track information. The ladar algorithm provides real-

time range, velocity, and acceleration tracks as well as a number of other diagnostic signals. Note its ability to see

acceleration jolts at the target turn-around points.

The LRL system is designed to precisely track satellites from a ground site. The system consists of a coherent,

doublet pulse lidar, a Kalman filter target tracker, and a feedback control system. The system has been thoroughly

tested in the lab environment. Key capabilities such as obtain high-accuracy tracks quickly and obtain real-time

processing and feedback control have been established. They include feedback control of processing mode, pulse

separation, pulse averaging, and data acquisition capabilities. The current algorithm provides real-time range,

velocity, and acceleration tracks as well as a number of other diagnostic signals. It has the ability to see acceleration

jolts at the target turn-around points. The prototype system was successfully tested at White Sands Missile Range

with static and spinning cone targets. Efforts are underway to carry out proof-of-concept experiments for ranging

and tracking of potential fast moving targets including International Space Station.

The concept of operation (Conops) for consists of a spectrally and spatially coherent laser beam that is cued and

directed towards the object of interest. The scattered laser light from the object is collected by a telescope where the

photons are detected and processed. In a bistatic arrangement, the incident and backscattered light utilize the same

optical telescope. Improved precision over existing sensor systems combined with mobility could benefit space

surveillance network operations. For our current ExoSPEAR lab system, performance models indicate the

achievable velocity resolution (10 ms pulse spacing) is < 1 cm/s, and the range resolution with a 10 ns pulsewidth is

~1.5 m.

3. PERFORMANCE MODELING

Using typical transceiver parameters, extensive performance modeling has been carried out. To illustrate the

anticipated velocity and acceleration precisions for ISTEF telescope facility located in NASA’s Kennedy Space

Center, Florida and AMOS telescope facility located in Mt. Haleakala, Maui, Hawaii were carried. In the case of

The ISTEF Site is located at the Kennedy Space Center, Florida. ISTEF is a Navy SSC PAC facility operated by

CSC. For ISTEF demonstration with a telescope aperture of 22 cm, a 100 W system with 10 Hz update provides a

velocity precision of ~1 cm/s upto a range of 2 megameters (Mm) and an acceleration precision of 0.1 m/s2 to 2

Mm. Similarly for Mt. Haleakala demonstration, using 400 W 50 cm system with 1 ms target coherence time and 10

Hz update, a velocity precision of 1 cm/s and an acceleration precision of <0.1 cm/s2, both upto 5 Mm is predicted.

Figure 5 illustrates rms velocity and ecceleration errors for Mt. Haleakala operation using typical parameters.

Figure 6 shows the clutter to noise ratio (CNR) estimation in dB versus range in Mm for various target sizes.

The current system is predicted to detect 10 m2 targets such as SeaSat & Okean beyond 1 Mm. With averaging of

100, one needs about -1 dB for reliable detection. The detection curve takes into account the automatic adjustment

of averaging level done by the system. The system will be able to detect 1 m2 targets within 500 km. Depending on

target altitude there should be ample signal to detect LAGEOS 1 (106 m2) by incorporating appropriate pointing

stability with no passive track.

Figure 5. Rms velocity and acceleration error predictions for Mt. Haleakala operation. Doublet pulse waveform

allows maintenance of range resolution and precision while significantly improving the velocity and

acceleration measurement.

Figure 6. Satellite CNR Predictions.

105

106

107

-10

-5

0

5

10

15

20

Range (m)

CN

R (

dB

)

CNR vs. Range

1 m2

10 m2

106 m

2

Target

4. ENGAGEMENT SIMULATION STUDIES

A system simulation has been developed to test processing and tracking algorithms. Real-time code was used

wherever possible. Realistic representation of total system has been developed. The key component to complete

the simulator is the signal generator. Accordingly, a signal simulator for realistic coherent data was successfully

developed. This signal simulator faithfully models speckle effects, including temporal correlation of phase and

intensity. The simulation allows for evaluation of tracking performance in various scenarios. For simulation noise

sources, local oscillator (LO) shot noise was included in signal simulator. Speckle effects were included in signal

simulator (and specification of target rotation rate). Accurate representation of amplitude modulation and velocity

spread were accomplished. However, TLO frequency drift, detector dark current, pointing errors and transmit pulse

phase and amplitude variations were not included. The engagement simulation uses a signal simulator to create

realistic coherent signals, models targets as collection of point scatterers and tracks each scatterer position and

velocity. Speckle effects were faithfully created. The output of signal simulator is simulated digitizer data which

includes shot noise effects. Figure 7 illustrates the top level tracking architecture. The signal generator produces

realistic raw data to simulate the DAQ subsystem

Figure 7. Top Level Tracking Architecture. The simulator includes all of the major subsystems.

Several scenarios were investigated. In one test case, the typical simulation parameters were as follows:

Engagement duration is equal to 100 sec. Lambertian sphere target of diameter of 4 m is considered. The LRCS of

10 m2 representing a more realistic target was considered. Rotation rate is equal to 100 mrad/s. High rotation rates

are considered to decrease speckle decorrelation time. Estimate decorrelation time is on the order of 6 µs. In this

case, angular scan pattern was implemented. A pointing jitter of 1 µrad, 1 σ on each axis was introduced. This was

independent between transmit and receive (due to time-of-flight). Figure 8 shows the simulation results. We see

good tracking in range, velocity, and acceleration, at least, at gross levels. The system chooses pulse separation as

large as possible without causing too much speckle noise. Initially sees too much speckle noise at ~40 s with a

spacing of 1280 ns. It held this level for over 20 seconds until it decided it produced too much noise. The system

reduces spacing and then uses smaller steps to increase doublet spacing and settles on 1134 ns as optimal spacing.

Figure 8. Simulation Results: Estimated Range, Velocity, Acceleration, Doublet Spacing and Doublet Angle.

Figure 9 illustrates simulation results of velocity and acceleration errors and scan pattern. The tracker produces

velocity estimates within ~2 cm/s of truth. This is an excellent performance since speckle is limiting the doublet

pulse spacing. Furthermore, acceleration is also tracked very well. In this case, better than 0.02 m/s2 is predicted.

Note that there is no direct measurement of acceleration, but derived from velocity measurements. In the case of

scan pattern simulation results, the coarse scan completes one petal before initial detection. The plot shows

commanded pointing angles, not actual achieved pointing angles. Jitter corrupts actual angle. Even in the presence

of jitter, the system finds the target and maintains pointing. Similar results in the other parameters, as well, have

been obtained.

0 20 40 60 801

1.05

1.1

1.15

1.2

1.25

1.3x 10

6

Time (s)

m

Filtered Range

0 20 40 60 80-3200

-3000

-2800

-2600

-2400

-2200

-2000

Time (s)

m/s

Filtered LOS Velocity

0 20 40 60 802

4

6

8

10

12

14

Time (s)

m/s

2

Filtered LOS Acceleration

0 20 40 60 8010

1

102

103

104

105

Time (s)

ns

Doublet Spacing

0 20 40 60 80 100-0.05

0

0.05

Time (s)

m/s

Velocity Tracking Error

0 20 40 60 80 100-0.05

0

0.05

Time (s)

m/s

2Acceleration Tracking Error

-5 0 5 10

-4

-2

0

2

4

6

8

rad

ra

d

Angular Pointing

Figure 9. Simulation results of velocity and acceleration tracking errors (top) and scan pattern (bottom).

5. SUMMARY, CONCLUSIONS AND PROGNOSIS

ExoSPEAR is an innovative waveform (pulse-doublet) coherent detection laser radar technology that enables

tracking of RSOs for unambiguous subsequent conjunction management and/or mitigation, and an architecture that

provides capabilities which have been difficult or impossible to achieve with existing passive or active sensors, and

offers unprecedented precise orbit determination, day or night. It offers a technology path for demonstrations in

long range precision ranging, tracking, vibrometry, and 3D imaging. The ExoSPEAR system, integrated with the

derivatives of the large space optics would provide for the rapid and precise detection and tracking, and offer

unprecedented remote sensing capability for a variety of exploration, science and technology demonstration

operations. The technology discriminator is the coherence pulse doublet for achieving improved track accuracies

with simultaneous mm class range precision, and mm/s class velocity precision. The operational discriminator

would be global mobility for ubiquitous orbital coverage LEO and beyond. Programmatically, operationally and

technologically, ExoSPEAR is as an integrated system of coherent doublet pulse ladar, coude-path optics, telescope,

ground and space operations compatible platforms, and logistics vehicles that ushers in a new spectral dimension for

space situational awareness measurements.

Modeling and simulation results are very promising. Actual algorithms used throughout simulation, except for

signal simulator. Excellent velocity track (~1 cm/s). Simulated system is able to effectively decide on processing

parameters in real-time. Various scenarios including singlet vs. doublet, averaging levels, and doublet spacing have

been investigated. System maintains the correct velocity ambiguity interval even as the waveform zooms in to 5

cm/s intervals and the target velocity is changing by 2 km/s.

The current prototype provides a framework and a test bed for precision ranging and tracking experiments of

RSOs in LEO. Preliminary models have indicated that the current ExoSPEAR system parameters could provide

tracking of targets in LEO. The ongoing planning emphasizes a conex based mobile based remote sensing system to

utilize established telescope sites data collection. The current system is suitable for ground-based tests at telescope

facilities including ISTEF facility located in NASA’s KSC and AMOS facility on Mt. Haleakala in Hawaii. The

ladar capability would provide for tracking, identification, classifying, characterizing and dynamically orienting

RSOs with unprecedented speed of acquisition, accuracy and resolution. ExoSPEAR could operate independently

and interdependently as a build-out of the Space Surveillance Network architecture. Field campaigns are being

planned. Finally, the proposed ExoSPEAR technology would populate remote sensing space situational awareness

architecture to monitor space operations, conduct discriminating science, and advance new technology.

ACKNOWLEDGEMENTS

The authors wish to acknowledge David B. Founds, AFRL/RDTP Chief, Systems Support Office, Kirtland AFB, Dr.

Duane Smith, (currently at Raytheon), Robert Kerns, Manager, Commercial Space Projects Office, NASA LaRC ,

William C. Edwards, ED, NASA LaRC for valuable contributions in setting up the ExoSPEAR system.

REFERENCES

1. See for example articles on orbital debris research in Orbital Debris Quarterly News (ODQN), NASA,

JSC, http://orbitaldebris.jsc.nasa.gov/newsletter/newsletter.html

2. Orbital Debris, A Technical Assessment, National Research Council, National Academic Press,

Washington D.C., 1995.

3. Barry Geldzahler et. al., "Coherent uplink arraying techniques for next generation orbital debris, near earth

object, and space situational awareness radar systems", in Active and Passive Signatures III, G. Charmaine

Gilbreath; Chadwick T. Hawley, Editors, Proceedings of SPIE Vol. 8382 (SPIE, Bellingham, WA 2012),

83820R.

4. See for example, references in “Orbital Debris Optical Measurements,”

http://orbitaldebris.jsc.nasa.gov/measure/optical.html

5. Narasimha S. Prasad, Albert DiMarcantonio and Van Rudd, “Development of Coherent Laser Radar for

Space Situational Awareness Applications, AMOS 2013, Sept 9-12, Maui, Hawaii.

6. Albert V. Jelalian, “Laser Radar Systems” Artech House, Inc., New York, 1991.

7. Takashi Fujii and Tetsuo Fakuchi (Editors), “Laser Remote Sensing”, CRC Press, June 2005.

8. Gary W. Kamerman (Editor), “Selected Papers on Laser Radar”, SPIE Press, 1997.